Prose

"Moreover, the Lord hardened the heart of Pharaoh, so that he did not listen to them, just as the Lord had spoken to Moses."
- Exodus 9:12, MEV

Why did God hardened Pharaoh's heart? Because he committed the only unforgivable sin - to resist and blaspheme (i.e. to be defiantly irreverent against) the Holy Spirit (Mark 3:29), in that after all the demonstration of God's wonder working power, he still chose to harden his heart. And because God knows that Pharaoh is adamantly not willing to repent, God gave him over to his heart's adamant defiance.
 
Throughout the Scriptures, we can find other references of God giving people over to their sinful ways because they harden their hearts even after God had called them to repentance for so many times (Psalm 81:10-12, Romans 1:21-24). There is even a record of Jesus condemning some towns to eternal damnation for being unrepentant even after all the demonstration of God's power that He showed them (Luke 10:11-14).
 
Like what C. S. Lewis once said, "There are only two kinds of people in the end: those who say to God, 'Thy will be done' and those to whom God says in the end, 'Thy will be done.'"
 
"Therefore, as the Holy Spirit says: 'Today, if you hear His voice, do not harden your hearts as in the rebellion, on the day of temptation in the wilderness, where your fathers tested Me and tried Me and saw My works for forty years. Therefore I was angry with that generation, and said, ‘They always go astray in their heart, and they have not known My ways.’ So I swore in My wrath, ‘They shall not enter My rest.’"
- Hebrews 3:7-11, MEV